Dispersants have been used in the construction industry, either singularly or as part of water-reducing compositions, to disperse cementitious mixtures allowing for a reduction in mix water content while maintaining flowability and workability of the mixture. This reduction in the water cement ratio leads to increases in compressive strength and is one of the main reasons that water-reducing admixtures are used. Compounds such as sulfonated melamine formaldehyde condensate (SMF), sulfonated naphthalene formaldehyde condensate (BNS), and lignosulfonates are commonly used as dispersants. However, these compounds are best suited for specific tasks. BNS and SMF are particularly difficult to use reliably and cost effectively at low levels and are best suited for use as high-range water reducers (greater than 12% water-reduction). Lignosulfonates in general tend to be best suited for lower water-reduction levels (less than 15%) and can cause excessive set retardation when used at higher amounts. Other materials such as salts of hydroxycarboxylic acids and sugars such as glucose or sucrose can also provide some degree of water reduction. In addition to the water reduction, the hydroxycarboxylic acids and sugars have commonly been used to retard the rate of set, which can lead to further improvements in compressive strength.
Dispersants such as BNS or lignosulfonates are often combined with additional components like sugars to achieve improved strength performance. These compositions usually must also contain accelerating components to offset excessive retardation. Even in combination with accelerating type components, formulated water reducers such as these can still retard excessively when used across a wide water reduction range in concrete mixtures containing pozzolans such as fly ash or slag, or in concrete that is mixed and placed at cool temperatures (50° F. or below). Additional accelerating admixtures are sometimes needed in an effort to offset this excessive retardation and depending on the severity, can be minimally effective. Excessive retardation is undesirable in that it can delay jobsite activity, prevent forms from being stripped, delay finishing operations or lead to low early age strengths. Providing an admixture with full range (Type A to F) water reducing capability and improved compressive strength while maintaining normal setting or easily controllable setting characteristics is desirable.
One improvement in the prior art was to use polycarboxylate dispersants over the conventional dispersants such as BNS and SMF. Polycarboxylate dispersants are structured with a polymeric backbone, such as a carbon chain backbone, with pendant moieties that provide the dispersing capabilities of the molecule. For example, polyacrylic acid has carboxylic groups attached to the backbone. Additionally, side chain moieties such as polyoxyalkylenes can be attached to the carboxylic groups to provide further dispersing capabilities. These polymers attach to the cement grains and produce dispersion by means of both electrostatic repulsion and steric hindrance, resulting in increased fluidity.
It is desirable to provide an admixture comprising a polycarboxylate dispersant that improves the compressive strength of cementitious compositions without exponentially increasing the set time, as is observed with BNS and lignosulfonate dispersants. Therefore, an admixture which improves the compressive strength of the hardened cementitious composition without producing any other changes would be advantageous in the industry.
U.S. Pat. No. 4,401,472 to Gerber discloses an additive comprising a poly(hydroxyalkylated) polyethyleneamine or a poly(hydroxyalkylated)polyethyleneimine or mixtures thereof, wherein the additive is present in a hydraulic cement mix in an amount sufficient to increase the compressive strength of the hardened mix.
U.S. Pat. No. 4,519,842 to Gerber discloses a cement mix comprising an admixture of poly(hydroxyalkylated)polyamine, alkoxylated poly(hydroxyalkylated)polyamine hydroxyalkylated derivatives of the compounds hydrazine, 1, 2, diaminopropane and polyglycoldiamine and mixtures thereof, wherein the admixture is present in amounts sufficient to increase the compressive strength of the hardened cement mix.